Abstract:

Fatty acids, being energy-rich compounds, are useful for understanding the transfer of energy from primary production to secondary production. One of the factors affecting the energy value of phytoplankton as a food source in aquatic food chains is a species of polyunsaturated fatty acid (PUFA) content. Polyunsaturated fatty acids, especially eicosapentaesnoic acid (EPA), play a key role in the transfer of energy in aquatic ecosystems. A low EPA content results in a low efficiency of energy transfer between phytoplankton and zooplankton which can contribute to the formation of algal blooms. Phaeocystis globosa and Microcystis aeruginosa are two common species that cause blooms in marine and freshwater systems. The fatty acid composition of cultured P. globosa HK and two strains of M. aeruginosa WN1 and WN2, were measured during their logarithmic and stationary growth phases by gas chromatography-mass spectrophotometry. These three strains of algae have simple fatty acid compositions, and their carbon chain lengths of fatty acids varied from 14 to 20. Saturated fatty acids are the dominant class, but the two essential fatty acids, EPA and docosahexaenoic acid (DHA), were not detected in the three strains. The total fatty acid content varied from 247.294 μg·g^-1 DW to 735.44 μg·g^-1 DW in P. globosa, C14:0 and C16:0 were dominant in the logarithmic and stationary phases, respectively. The total fatty acid content was much higher inM. aeruginosa WN1 and WN2, ranging from 1 405.095 μg·g^-1 DW to 6 087.617 μg·g^-1 DW weight, and C16:0 dominated both growth phases for the two strains. There was a significant difference in the content of fatty acid on a dry weight basis between the two growth phases for both strains ofM. aeruginosa. This indicated that the composition of algal fatty acid changed with growth phase. However, this difference did not occur in P. globosa. Due to the absence of EPA and DHA in these two species, both P. globosa and M. aeruginosa cannot provide the essential fatty acids to grazers at higher tropic levels, such as Daphnia. Because of the poor food quality, these two species are not grazed by zooplankton, which most likely contributes to their noxious quality of forming algal blooms.

Key words: Algal blooms, Fatty acid composition, Phaeocystis globosa, Microcystis aeruginosa